Many current or future electronic applications are being or will be required to function at higher and higher frequencies. These applications are not limited to the telecommunications market. Switches in high-frequency ranges are also required for vehicle-borne electronics intended for automotive and ground-based means of transportation, for aeronautics, and for medical systems or in-home automation, for example. These applications, for the most part, require high-power switches, typically switching between 500 V and several kilovolts, with currents most often between 10 and 200 A, functioning in frequency ranges above one megahertz.
Historically, high-frequency power switches have used field-effect transistors based on a semiconductor channel. At lower frequencies, junction transistors (thyristors, etc.) are preferred because they are able to withstand higher current densities. However, because of the relatively limited breakdown voltage of each of these transistors, power applications require many transistors to be used in series. These series transistors generate substantial losses, both in the steady-state and switching regimes.
Alternatively, high electron mobility transistors (HEMTs), also denoted by the term heterostructure field-effect transistors (HFETs), may be used in high-frequency power switches. Such transistors include a superposition of two semiconductor layers having different bandgaps, forming a quantum well at their interface. Electrons are confined to this quantum well and form a two-dimensional electron gas. To withstand high-voltage and temperature, these transistors will be chosen to have a wide bandgap.
Among wide bandgap HEMT transistors, those based on gallium nitride are very promising. The width of their bandgap results in a higher avalanche voltage, compared to conventional electronic materials (Si, SiGe, GaAs, InP), in a high carrier saturation velocity, and in good thermal and chemical stability (enabling use in extreme environments). The breakdown field of gallium nitride (GaN) may thus be higher than 3×106 V/cm, thereby easily allowing transistors with breakdown voltages higher than 100 V to be produced (300 nm of GaN is sufficient). In addition, such transistors allow very high current densities to be obtained because of the high electron mobility inside the interface gas.
Gallium nitride has a bandgap width of 3.39 eV. In addition, ternary alloys such as AlGaN or InGaN may easily be produced in a tool for forming GaN by epitaxy. A HEMT transistor based on gallium nitride may also be produced on a silicon substrate (which is the workhorse substrate of the semiconductor industry). GaN HEMT transistors are therefore much less expensive to produce than transistors based on SiC for example. Even though SiC-based transistors also have a wide bandgap, ternary alloys are difficult to produce with this material, and it cannot be grown on a standard silicon substrate, thereby making its fabrication cost prohibitive and greatly limiting its applications.
The document published by M. Tatsuo Morita entitled “650 V 3.1 mΩ.cm2 GaN-based monolithic bidirectional switch using normally-off gate injection transistor” pages 865-868 of the proceedings of the IEEE conference “Electron Devices Meeting 2007,” describes a monolithic bidirectional switch comprising two AlGaN/GaN normally-off transistors connected back-to-back with a common drain. Switching of these transistors is controlled by way of respective gates. The voltage of each gate is referenced to the source potential of the corresponding transistor. Therefore, the gate voltages are referenced to different potentials. The control circuit of the gates is thus complicated to produce, in particular for such a frequency level.
U.S. Pat. No. 7,982,240 provides a solution allowing a single control signal to be applied to the gate of a single normally-on transistor. Two Schottky diodes are here connected in series between the source and drain of the transistor. The diodes have common anodes, one cathode is connected to the source of the transistor, and the other cathode is connected to the drain of the transistor. The source and drain functions of the transistor are in practice inverted depending on the flow direction of the current. The connection node of the anodes of the diodes delivers a reference potential in order to allow the gate of the transistor to be biased relative to this reference. The potential of this reference changes depending on the direction of the current flowing through the switch.
However, such a structure also has drawbacks. The diodes must be dimensioned for high voltages. The addition of such diodes implies the addition of etching masks during the fabrication process.